Well, like I mentioned in this news post, NAS boxes go beyond the hardware, and I would have little doubt that the software is responsible for a rather sizable chunk of the total price. The KISS mentality works well here, because rather than have folks do everything manually (or write their own scripts), they can utilize the software on the NAS box to take care of things for them.
NAS boxes also have the benefit of not being connected to any particular computer. Rather, they are always available through your home network, so you can grab the media you need from any PC or network-enabled device (such as an Xbox 360).
Most robust NAS boxes (this one included, I'm sure), would feature sufficient cooling for long-life, and also a removable filter that would be used to catch dust.
Personally, I use NAS boxes for the extra security. I don't think anything I have data-wise is truly important (well, except family photos and the like), but I like to make absolute certain that I never lose any of it. So, I have a secondary HDD in my main machine that backs up all my documents, and then the NAS comes into play and grabs all of the same data. At any given time, I have three copies of my data, four if you count the selected data that I burn to DVDs.
In the end, ease-of-use is king with NAS boxes. If you build your own PC as a server, it takes a lot more work and technical know-how to get anything going, and most people aren't going to want to put in the effort. One thing I wouldn't mind doing though, is building an HTPC that also acts as a NAS box. That way, you get two machines in one, essentially, and it's easy to fit HUGE amounts of storage in an HTPC nowadays, even if you want to use RAID.